Carbooxetanocins such as 9-[(1R, 2R, 3S)-2,3-bis(hydroxymethyl)-1-cyclobutyl)guanine, which are disclosed in EP-0358154-A2 and EP-0366-59-A2, are expected to be usable as antiviral agents. The aforesaid patents further disclose an optically active substances of a compound represented by the general formula [I], which will be given hereinafter, wherein R.sup.1 and R.sup.2 are each a silyl type protective group, as an intermediate in the synthesis of the above-mentioned compound.
It is an object of the present invention to provide a novel process for producing an optically active substances of a compound represented by the general formula [I].